Interactions between spacecraft and a space plasma environment can cause large electrical potential differences to develop between spacecraft structures and the space plasma environment. If uncontrolled, the charging of the spacecraft can produce enhanced surface contamination, increased power leakage currents, and electrical arcs that cause system performance degradation or failure.
Currently available spacecraft charging detection and measurement instruments include floating potential probes, charged-particle energy spectrum analyzers, and electrostatic surface potential monitors. Active charge control techniques include thermionic electron emitters, neutral gas release systems, and plasma generators. A typical charge control device is generally referred to a plasma contactor. This device employs an electrically driven hollow cathode system to ionize and then release an expendable gas such as argon, xenon, or krypton from an onboard gas supply tank. The resulting plasma cloud, which is generated essentially at the spacecraft""s potential, allows charged particles of the appropriate polarity to flow to the ambient plasma environment, thus reducing the absolute spacecraft potential.
Disadvantages with this type of charge control device include the weight and complexities associated with the gas storage and mechanical gas flow control subsystem and the time limits placed on operating by the finite gas supply. Also, to maximize system effectiveness and prevent the unnecessary use of gas, a separate supplementary charging monitor system is required to determine when to turn the plasma contactor on and off. Thus, there remains a need for a system and method for controlling charge on a spacecraft without the disadvantages of the devices described above.
One objective of the invention is to provide a charge emitter that is suitable for use in space applications, i.e., a charge emitter that is lightweight, small in size, and reliable and robust in the space plasma environment. Another objective is to provide a charge emitter capable of emitting low levels of charge, positive or negative, for controlling the charging of a space object. Another objective is to provide a charge emitter capable of emitting large levels of charge, positive or negative, for use in adjusting the orbit of space objects. Yet another objective is that the charge emitter be capable of emitting charge without requiring high voltages. Still yet another objective is that the charge emitter be capable of emitting electrons without the use of expendable resources.
The invention features a sensor for measuring an electrical potential difference. The sensor comprises a field emission device, situated in an environment, having an emitter and a gate. A voltage source applies a voltage across the gate and the emitter to induce current to flow from the emitter. The magnitude of the applied voltage can be less than 100 volts. A control system in communication with the field emission device obtains measurements of the current flowing from the emitter and of current flowing to the gate. The control system also determines an electrical potential difference between the field emission devise and the environment from the gate current and emitter current measurements.
In one embodiment, the control system comprises a processor that determines the electrical potential difference between the field emission device and the environment from a functional relationship between the gate and emission current measurements. The field emission device can be electrically connected to an object. Accordingly, the electrical potential difference measured by the control system corresponds to an electrical potential difference of the object with respect to the environment. The control system can compute the electrical potential difference from the current measurements. In one embodiment, a table can store correspondences between a functional relationship of the gate and emitter currents and electrical potential differences of the field emission device with respect to the environment. The control system can use this table to determine the electrical potential difference from the current measurements.
In another embodiment, the control system comprises a voltage controller in communication with the voltage source to control the voltage applied by the voltage source between the gate and the emitter. The voltage controller alters the flow of current from the emitter by adjusting the applied voltage so as to maintain the electrical potential difference between the field emission device and the environment at a predetermined voltage.
The gate and the emitter of the field emission device have a spatial relationship that facilitates the determination of the electrical potential difference between the field emission device and the external environment of the field emission device. The spatial relationship of the gate and emitter induces at least a predetermined amount of current to flow to the gate.
In another aspect, the invention features a sensor for measuring an electrical potential difference that comprises a field emission device situated in an environment and having an emitter and a gate. A voltage source applies a voltage across the gate and the emitter to induce current to flow from the emitter. A first current monitor in communication with the emitter measures current flowing from the emitter in response to the applied voltage. A second current monitor in communication with the gate measures current flowing to the gate. A control system in communication with the voltage source and current monitors determines an electrical potential difference between the field emission device and the environment from the gate and emitter current measurements.
In yet another aspect, the invention features a controller for controlling an electrical potential difference of a space object with respect to an environment of the space object. The controller includes a field emission device with appropriate exposure to the environment of the space object. The field emission device has at least two terminals and is electrically connected to the space object. A voltage source applies a voltage across two terminals of the field emission device to induce current obtained from the space object to flow from one of the terminals into the environment of the space object. A control system measures an electrical potential difference between the field emission device and the environment of the space object, based on measurements of currents flowing through the two terminals. The control system alters the flow of the current from the emitting terminal by adjusting the applied voltage so as to control that electrical potential difference.
In still another aspect, the invention features a method for measuring an electrical potential difference. A voltage is applied across a gate and an emitter of a field emission device situated in an environment to induce current to flow from the emitter. Measurements of the emitter current flowing from the emitter and of current flowing to the gate are obtained. An electrical potential difference between the field emission device and the environment is determined from the measured gate and emitter currents. In one embodiment, the applied voltage is adjusted to adjust the emitter current and to maintain the electrical potential difference between the field emission device and the environment at a predetermined voltage. The field emission device can be connected to an object. The determined electrical potential difference thus corresponds to an electrical potential difference of the object with respect to the environment.
In another aspect, the invention features a method for sensing and controlling an electrical potential difference of a space object with respect to an environment of the space object. An at least two-terminal field emission device has appropriate exposure to the environment of the space object and is electrically connected to the space object. A voltage is applied across two terminals of the field emission device to induce charge obtained from the space object to flow from one of the terminals into the environment of the space object. An electrical potential difference is measured between the field emission device and the environment of the space object based on currents that are measured in response to the applied voltage. The flow of the charge from the emitting terminal is altered by adjusting the applied voltage, to control the electrical potential difference between the space object and the environment of the space object.